[QUOTE/boomana] I also know one person who recently sold his ZD, not because he didn't love it, but the Woo WA5 came out and gave him everything he wanted, including the ability to consolidate his speaker/headphone rigs. I doubt he'll be selling that any time soon. So, maybe I'm wrong, and people like what they like when they like it and then buy and sell when the mood strikes. Who knows, maybe I'm next.

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That is pretty much the reason people sell what they have. I have sold dozens of some of the best audio gear made(both headphone and speaker) over the years. Not because they didnt sound great or work right...but because my system was changing and, or room size was changed. The Singlepower amps (along with many others for sale used) are VERY, VERY good. One mans loss is another mans gain. So of all the reasons for selling in this thread the one in your post that says..
People like what they like when they like it and then buy and sell when the mood strikes. Holds the most true.
In the first five pages of amps for sale there are
11 headroom
10 Ray Samuels
10 Mier
9 Singlepower
7 Headamp
amps for sale. No magic reasons. People either dont use them, found something they like more, need the money, or whatever.
They like what they like when they like it and then buy and sell when the mood strikes. If I was in the market for a headphone amp I wouldnt be wondering why the amps from these great builders are for sale....but how I can come up with the money to get one at used prices.